BROOKLYN, Iowa -- New details in the Mollie Tibbetts case indicate that she was stabbed to death, and new details reveal that it's possible the suspect's girlfriend knew Tibbetts.

Mollie Tibbetts died from "multiple sharp force injuries," according to a preliminary autopsy report released by the Iowa State Medical Examiner Thursday. This would indicate she died after being stabbed, ABC News reports.

Tibbetts' death was officially ruled a homicide, and Cristhian Bahena Rivera has been charged with first-degree murder.

New details continue to come to light in the case. Tibbetts attended the same high school as the suspect's girlfriend, Iris Monarrez, ABC News learned on Friday morning. It appears the two were friends on Facebook, but investigators stated that this does not mean they were personally acquainted. Because Brooklyn, Iowa, is such a small town, they may have connected simply because they went to the same school.

Earlier this week, investigators said they were uncertain how Tibbetts was killed or whether she was sexually assaulted. They've made no mention of recovering a weapon linked to the death.

The examiner did not release additional details about the injuries Tibbetts suffered or what caused them, but said further examination of her body may result in additional findings. Autopsy reports are confidential under Iowa law, except for the cause and manner of death.

Tibbetts was last seen July 18 in Brooklyn, a rural town of about 1,400 people, as she took a routine evening jog. It's unclear whether she returned to the home where she was dog-sitting for her boyfriend and her boyfriend's brother, who have said they were both out of town. She was reported missing by her family the next day after she didn't show up for work at a day camp for children.